1. AbbottD. C. 1982, Ap. J., 263, 723.
2. Aller L. H. 1987, in Physics of Thermal Gaseous Nebulae (Physical Processes in Gaseous Nebulae), (Dordrecht: Reidel), Vol. 112, p. 101.
3. BeckerW. and Fenkart R. B. 1970, in IAU Symposium No. 38, The Spiral Structure of Our Galaxy, eds. W. Becker and G. Contopoulos (Dordrecht: Reidel), p. 205.
4. Brand P. W. J. L. and Mathis J. S. 1978, Ap. J., 223, 161.
5. Bregman J. N. and Harrington J. P. 1986, Ap. J., 309, 833.